302 



W. M. BALDWIN. 



direction. Because of the condition of fixity of the bend pro- 

 gression directly forward is necessarily out of the question. 



The deformity represented by Fig. 6 is illustrative of the second 

 mechanism of production of the folded embryos. The concave 

 side of the neural tube, to the right in the figure, is occupied 

 by irregular bulgings of the epidermal surface of the embryo. 

 When the development of embryos conforming to this type is 

 followed from day to day, it is noted that the flexure of the neural 

 tube becomes greater from day to day. The apparent occasion 



FIG. 6. 



of this is referable chiefly to two factors; one, the normal but 

 relatively more rapid growth of the normal myomeres to the 

 left of the longitudinal axis of the body, and the other, to the 

 relatively, and actually slower, growth of them upon the concave 

 side of the median body plane. Histological studies demon- 

 strated as well that differentiation in the myomeres upon the 

 affected side was somewhat retarded. 



There are, therefore, these two general developmental mechan- 

 isms either of which leads to the production of a U-shaped 

 embryo. In one the angularity of the bend is constant from the 

 time of its first appearance in the embryo, whereas in the other 

 this angularity increases with the succeeding stages of differentia- 

 tion and growth of the anlagen. The end result is identical, 

 however, in both instances both from the morphological and the 

 histological standpoint. Furthermore, in both instances the seg- 

 mental relationship of the bend to the area of protoplasm altered 

 in the ovum is the same. 



